Ship the side-highlight prototype: a rail means nesting now

[jonathanKingston]

prototypes/side-highlight (#2350) landed as prototype files only — 3,138 lines of
workshop, and nothing in src/. Its inventory of ten accent rails still matched the
source line-for-line, so none of its proposal had been implemented. This implements
M — the mix, the design it proposed.

The design

The rail used to do three unrelated jobs, and looked the same doing all of them:
containment in the transcript, selection in the sidebar, nesting under a tool card.
One device serving three meanings is what made it read as repetition rather than as
signal. Only nesting keeps it, which is literally what a nesting guide depicts.

Block	Treatment
GitHub alerts, blockquotes	flat plate — prose the agent wrote, part of the answer
Thinking, review, comparison	hatched plate — Copse annotating its own turn
VNC pane	gutter — its status hue has to survive on one line, where a plate would just box three of them
Tool rollup, subagent timeline	unchanged — structure, not decoration
Thread rows, Settings nav, VNC tabs	fill alone — the full-bleed wash already says "current"

Alerts and blockquotes are styled by @copse/streaming-markdown, so they are
overridden locally in styles/global/markdown.css: renderer/main.ts imports the
package's default.css before global.css, so rules at the package's own specificity
win on source order. No upstream change was needed.

The prototype's open questions

• Moiré — settled by the number. A repeating gradient beats against the pixel grid
  when its period lands near an integer count of device pixels. 8 is a multiple of 4,
  so at every quarter-step scale Windows offers (1.25×, 1.5×, 1.75×, 2×) the period is a
  whole number of device pixels and there is no beat frequency. The pitch is now load
  bearing; base.css says so where it is declared.
• Reduced transparency / high contrast — done. The hatch lines go to nothing and the
  wash comes up to the plate's weight, so commentary keeps a surface and loses only its
  distinction from content.
• Two new hues. Five alert kinds need five and the palette had three. --info and
  --important join the semantic tokens with derived light values. --important is
  pushed bluer than a true violet so it does not read as the pink accent — a user
  whose custom accent is itself purple still collapses that, which is written down
  rather than fixed (the fix is to derive --important away from --accent).
• Callout glyphs are solid — the only filled shapes in an otherwise outline set, so
  docs/ui-taste.md now states the house rule outright: filled marks transcript status,
  outline is chrome. That is also what scopes the two warning triangles rather than
  retiring one. The optical-centring nudges are baked into each mask's viewBox,
  measured once by the prototype and not recomputed at runtime.

For a reviewer

• The bug worth knowing about. The finished backgrounds were first put in shared
  custom properties. A custom property is substituted where it is declared, so
  --callout-plate froze :root's grey into its computed value and inherited that grey
  everywhere — every plate came out the same neutral. A screenshot did not catch it; the
  new callout-surfaces e2e spec did. The tuning now lives in base.css and the colour
  is mixed at each use site.
• .thread-proposal keeps its rail. It landed in Let the agent propose whole new threads #2334, after this inventory was
  taken, and its rail marks a standing ask — a fourth job the bar was not doing when
  the prototype surveyed it. Allowlisted with the reason rather than converted silently.
  Worth deciding deliberately.
• accent-rails.test.ts now holds the list closed. It kept the "a rail never curves"
  contract and gained a second rule: a rail may only mean nesting. A trailing-edge
  border is out of scope there, because that is how the app walls one pane off from
  the next.
• docs/ui-taste.md contradicted this in three places — "Accent rails never curve",
  "Sidebar selections", and "Transcript status callouts", which used to forbid a tinted
  block outright. All three are rewritten with the reasoning, not just the new rule.

Verification

• pnpm run check passes end to end (8544 pass / 0 fail / 7 skipped).
• New focused spec tests/e2e/callout-surfaces.e2e.ts seeds every affected surface on
  one screen and asserts the plate resolves per severity, the hatch actually hatches, the
  selected row has neither border nor shadow, and all five glyphs are distinct — with
  dark and light workbench screenshots.
• Strong forest tint checked by hand: the plate and hatch are color-mix over live
  theme tokens, so they follow it without re-derivation.
• The 536 committed reference screenshots are stale. check:screenshots flags them,
  and the sanctioned path is the CI e2e candidate artifact — hence the
  update-screenshots and ci-full labels on this PR.

🤖 Generated with Claude Code

[github-actions]

🖥️ PR preview

• Marketing site
• Browser demo
  Interactive, with a mocked backend — jump to a scenario:
• Builds a cupcake site
• Agent-proposed edits open the Changes panel
• Markdown list indentation
• Responsive composer footer
• Subagent display visual reference
• Subagent stays ungrouped beside reading tools
• Semantic search subagent markdown
• Light-theme approval with a bright accent
• Light-theme syntax, fills, and selection
• Grouped outside-sandbox command approval
• Remote desktop discovered-port list with one selected
• Settings scroll + sticky footer geometry
• Per-model reasoning / temperature / top-p
• Thread switch hydration notice
• Thread switch hydration failure
• Chat layout styling
  _{Static demo of the renderer against a mock API — the browser pane, terminal, and all main-process IPC are stubbed, so it never proves main-process behaviour. Rebuilt whenever a push changes the demo bundle; these links go live once the queued deploy lands, shortly after this comment. Removed when the PR closes.}

[copse-release-bot]

Reference screenshots ready for review

Review GitHub’s image diffs in screenshot PR #2572.
Merge it (or enable auto-merge) to apply the accepted PNGs to claude/copse-threads-prototype-c5c83c.

Rendered for 2c1eb06c756b by CI run 34157525663; the immutable artifact is reference-screenshot-candidates-34157525663.

If this source branch moves, a later successful render closes the stale review PR and replaces this link.